A known hydraulic pressure control system for an automatic transmission forms clutch pressure by directly controlling the hydraulic pressure from hydraulic pressure source with a solenoid valve without adopting an accumulator. With the foregoing known hydraulic pressure control system, the following method is proposed for achieving the smooth and high responsive gearshift feeling by swiftly supplying oil to frictional engagement elements (i.e., frictional clutches, frictional brakes) at shifting. According to the known method, the communication between the frictional engagement elements and line pressure is established at the start of the hydraulic pressure supply, and the direct communication between the frictional engagement elements and the line pressure is blocked when the supply of the hydraulic pressure is completed to hold at pressure corresponding to the pressure of return spring by establishing the communication between the frictional engagement elements and a pressure control valve, that is, the method for transiting to the pressure control after the flow control (i.e., precharge control).
With this regard, because the flow control depends on the individual differences deriving from the movement of a clutch piston, the input and the output volume of the oil, and the leakages of the oil, or the like, and also varies depending on the piece-to-piece individual variations such as the automatic transmission and the engine, a method for compensating for individual differences is desired at the initial state for shipment. It has to be considered that the determination of the hydraulic pressure characteristic value at the initial state for shipment is conducted at the vehicle stop state without running the vehicle, and all conditions for actual shift cannot be reproduced. Particularly, a large volume of the operational fluid is consumed when switching the oil path at the shift and when the transmission is controlled to charge the fluid suddenly at the initial stage of the shift, and the operational fluid is unlikely sufficiently supplied by the pump output volume for the determination of the precharge time, as described in Japanese Patent Laid-Open Publication JP10(1998)-96466A2 and Japanese Patent Laid-Open Publication JPH10(1998)-68462A2. Thus, the decline of the line pressure influences on the tested value further than the piece-to-piece individual variations of the automatic transmission and the engine.
As shown in FIG. 12 showing the oil pump performance used for the automatic transmission, the pump output volume is increased proportional to the rotational speed of a pump. A variable volume type vane pump and a gear pump with flow control valve, or the like, is adopted for controlling the output volume to be minimal to restrain the excessive flow when the engine rotational speed reaches a predetermined rotational speed which corresponds to the unnecessary flow amount determined every automatic transmission. The pump output volume increases in accordance with the increase of the engine rotational speed within the range of the actually required oil amount.
Although it is recognized that the sufficient oil volume is not necessarily ensured at a low rotational range such as at the idling state with the pump of the foregoing kind, in the industry, the loss of the oil pump is apt to be reduced by reducing the size of the oil pump. Accordingly, in case of increasing the precharge pressure at the test for a “garage shift” driving such as the shift from the neutral range (i.e., N range) to the drive range (i.e., D range) and from the neutral range to the reverse range (i.e., R range), the line pressure is declined to largely vary the precharge time for every individual, which requires to decreases the precharge pressure within the range to restrain the influence of the decline of the line pressure.
FIG. 13 shows each oil pressure value during the test operation for obtaining the precharge time of the frictional engagement elements at the actual shift when the turbine rotation is at the rotational speed corresponding to the idling state and the variation of the turbine rotational speed. FIG. 14 shows each oil pressure value during the test operation for obtaining the precharge time of the same frictional engagement elements when the turbine rotation is at the rotational speed corresponding to the normal operation (i.e., normal shift) and the variation of the turbine rotational speed. The oil pressure characteristic value of the frictional engagement elements used at the normal shift may be determined along with the frictional engagement elements for the “garage shift” driving at the stage of initial state for shipment. However, as shown in FIGS. 13–14, in case the test of the oil pressure characteristic value of the frictional engagement elements used at the normal shift is conducted at the idling operation, the decline of the line pressure assumes larger, and the maximal precharge time (max) at the idling operation assumes longer than the maximum precharge time (max) tested at the approximate to the actual condition (shown in FIG. 14). Further, at the idling state, because the consumed flow amount cannot be sufficiently compensated by the pump output volume, the leakage at each portion highly influences to further increases the variations of individual differences of every automatic transmission, which provides the unstable test result. With this regard, although the variations may be restrained by determining the precharge pressure low likewise the test at the “garage shift” driving, it is not preferable to change the precharge pressure of the normal shift for the purpose of the test because the higher precharge pressure is applied at the normal shift in order to swiftly supply the oil to the clutch.
The test (i.e., the setting of the precharge time by learning) is conducted either by operating a throttle pedal by the tester or by the control by the engine. With the operation for learning requiring the operation of the throttle pedal by the operator, the safety of the engine rotation may not be necessarily ensured and producers and dealers may have more burden who have been having other operations for the adjustment requiring large number of man-hour. Further, the control by the engine may not be applicable to the all vehicles and may generate the periodical fluctuation of the engine, which generates further problems for the test guarantee.
A need thus exists for a method for a precharge time determination for automatic transmission and an automatic transmission provided with a determination function for the precharge time with the method thereof.